Brake fluid is hygroscopic which means it attracts and absorbs water. This is a natural occurrence in all brake systems. Contrary to a popular myth, there is no such thing as a sealed brake system. Because brake fluid attracts water like a magnet attracts metal, moisture can even be drawn into the fluid through the pores of rubber brake system components. Moisture in the brake system combines with available oxygen causing oxidation of brake hydraulic parts. Some cheap, some staggeringly expensive!